This invention relates to communications systems and a method for controlling such systems.
Systems providing for the transport of encoded signals on a communications link are well known. Many such systems are used for data signals which are not time critical; examples of such systems are packet switching networks and many local area networks (LANs). Many others can carry speech and video signals, where transmission delays above certain limits are unacceptable, but not data signals.
It has been proposed in our European Patent Application 85305017.7 published as 0168265 which corresponds to the applicant's above-identified U.S. Pat. No. 4,933,935 to provide a communications network as a ring with a number of stations and is capable of handling digital data from data processors, video or telephone equipment using a configuration that is known as a slotted ring protocol.
The present invention is concerned with modifications and improvements to such systems.
According to the invention there is provided a method for controlling a communications network comprising a plurality of stations which can communicate via a closed communication loop; said method including: providing information in digital form for transmission from one station to another in message blocks which circulate around the loop, each message block being provided as a first group of control bits and a second group of data bits, controlling each station when it requires to transmit so as to assume an active state in which it inserts digitally encoded information into empty blocks for transmission to a destination station via the loop, a station if it has inserted information into d blocks enters a state in which it cannot gain access to empty blocks for the transmission of information until it is reset in response to recognition of an empty block having travelled at least one around the loop, said resets being issued to each of said stations so that no station is in a preferential position with regard to the rate of reception of resets, and wherein the control bits of each block include bits which when that block is emptied at a station can then be set at that station to indicate a particular state indicative of a further allocation request.
According to a further aspect of the invention there is provided a communications system, comprising a plurality of locations for storing information to be carried in message blocks on a link, each location including control means for providing an allocation of d blocks to entitle the location to transmit one block onto the link whenever a block is waiting at that location, but if d blocks are transmitted onto the link from a location, that location loses its entitlement to transmit further blocks until a further allocation is received, and wherein further allocation means are provided for initiating a procedure for further allocation each time a block is emptied at any of said locations and is completed whenever a block initiated by said procedure travels throughout the link without cancellation by another location.
According to a further aspect there is provided a method of operating a communication system comprising allocating d message blocks only to each station within the system to avoid monopolisation by any station; utilising a block emptied at any location for the purpose of testing the allocation availability of further blocks regardless of whether the d allocation has been exhausted; and allocating further blocks whenever the utilised block has travelled throughout the system without being modified by another station.